This invention relates to a heat transfer sheet, more particularly a heat transfer sheet capable of giving easily recorded images excellence in various fastnesses to image-receiving materials.
Heretofore, various heat transfer methods have been known and among them, there has been practiced the sublimation transfer method in which a sublimable dye is used as the recording agent, this is carried on a substrate sheet such as paper, etc. to provide a heat transfer sheet, superposed on an image-receiving material dyable with a sublimable dye such as a fabric made of polyester, etc. Heat is applied in a pattern from the back surface of the heat transfer sheet, thereby migrating the sublimable dye to the image-receiving material.
In the above sublimation transfer method, in the sublimation printing method wherein the image-receiving material is, for example, a fabric made of polyester, etc., since heat energy is imparted for a relatively long time, the image-receiving material itself is also heated with the heat energy imparted, whereby relatively good migration of the dye is accomplished.
However, with the progress of the recording method, when by use of a thermal head, etc., fine letters, figures or photographic images are formed at high speed on, for example, polyester sheets or image-receiving materials having dye receiving layers provided on paper, a very short duration (seconds) of heat energy is required. Therefore, because the sublimable dye and the image-receiving material cannot be heated sufficiently, an image with sufficient density cannot be formed.
Accordingly, in order to respond to such high speed recording, a sublimable dye excellent in sublimability has been developed. However, a dye excellent in sublimability generally has a small molecular weight, and therefore the dye will be migrated with lapse of time in the image-receiving material after transfer, or bleed out onto the surface, whereby such problems occur that the image formed elaborately is disturbed or becomes indistinct or the surrounding articles are contaminated.
For avoiding such problems, if a sublimable dye having a relatively larger molecular weight is used, the sublimation rate is inferior in the high speed recording method as described above and hence no image with satisfactory density can be formed as described above.
Accordingly, in the heat transfer method by use of a sublimable dye, it has been strongly demanded under the present situation to develop a heat transfer sheet which can give sharp images with sufficient density and yet give images formed exhibiting excellent various fastnesses by imparting heat energy for an extremely short period of time as mentioned above.
The present inventors have studied intensively in order to respond to the strong demand in this field of the art as described above, and consequently found that, although in the sublimation printing method of the fabric made of polyester, etc., because of the surface of the fabric which was not smooth, the heat transfer sheet and the fabric which was the image-receiving material were not sufficiently contacted closely with each other. Therefore it has been essentially required that the dye used should be sublimable or vaporizable (namely the property of being migratable through the space existing between the heat transfer sheet and the fabric). In the case when a polyester sheet or converted paper, etc. having a smooth surface is used as the image-receiving material, only sublimability or vaporizability of the dye is not the absolutely necessary condition because the heat transfer sheet and the image-receiving material can be sufficiently contacted closely with each other. However, the property of the dye migratable through the interface of both the closely contacted by heat is also extremely important, and such heat migratability through the interface is greatly influenced by the chemical structure of the dye, substituents or the positions thereof. The present inventors have also found that even a dye having a high molecular weight the extent which has been considered to be unavailable according to the common sense in the prior art has good heat migratability by selection of a dye having an appropriate molecular structure. By use of a heat transfer sheet having such dye carried thereon, it has been found that the dye used can be easily migrated to the image-receiving material to form a recorded image having high density and excellent various fastnesses, to accomplish the present invention.